1. Field of the Invention
This invention relates in general to imaging devices, and in particular to solid state color imaging devices.
2. Background Relative to the Prior Art
The prior art, say, for color cameras, involves electron beam scanned tubes: A three color signal is derived by either utilizing three tubes with a beam splitter and optical filters or one tube with a color stripe filter affixed to the image receiving surface of the target. The former method requires the maintenance of registration of the image on the three separate tubes and the latter method suffers from loss of resolution, at least in part, because the stripe filter must be separated from the target by approximately 100 microns.
Recent U.S. patents, namely, U.S. Pat. Nos. 3,860,956 and 3,576,392, describe single beam scanned color image tubes which do not utilize color filters. The target of each tube is comprised of a plurality of photodiodes. The color imaging capability arises from the intrinsic wavelength dependent optical absorption of the target material, which in both cases is silicon. Blue light is more strongly absorbed than green light which is, in turn, more strongly absorbed than red light. This is termed differential optical absorption. The imagers described in '956 and '392 have their photodiodes grouped into pixel triads and are so constructed that each member of a triad has a different spectral sensitivity. For '956, the pixels are sensitive to blue (B), blue plus green (B+G), and blue plus green plus red (B+G+R). For '392, the pixels are sensitive to (R), (R+G) and (R+G+B).
Various techniques for providing a solid state color imaging device have started to appear in the literature. These solid state devices are based upon arrays such as charge coupled devices (CCD's), charge injection devices (CID's), photodiodes, and phototransistors, which are self-scanned as opposed to beam-scanned image tubes.
U.S. Pat. No. 3,971,065 to Bayer discloses one approach to implementation of such solid state arrays. The general approach of Bayer is by the use of special arrangement to triads of color filters overlaying the imaging sites. The color filter mosaic optimizes the resolution for a fixed number of image sites. A CCD imager incorporating this concept was reported by Dillon et al, International Electron Devices Meeting, Washington, D.C., December 1976.
Published Patent Application B-502,289 describes another solid state imager, such imager employing a color coding filter affixed to a solid state, self-scanned array.
A third approach to solid state color imaging, which approach utilizes the differential optical absorption of the silicon substrate to provide the three color signal, is described in U.S. Pat. No. 3,985,449. This approach employs adjacent pixel triads. As a result of different voltage biasing conditions the three pixels of a triad are sensitive to (B), (B+G) and (B+G+R), respectively.
Yet another approach to solid state color imaging is shown in U.S. Pat. No. 3,717,724. According to this approach, a sandwich comprised of a plurality of image sensing arrays is employed to generate a plurality of color signals. Each image sensing array in the sandwich is comprised of a semiconductor material adapted to selectively absorb a certain portion of the electromagnetic spectrum. Associated with each image sensing array is a plurality of contact elements arranged for retrieving the photosignals generated by the array.